The effect of inulin and fulvic acid, added in the feed, on growth, survival, WSSV prevalence, and immune system was studied in Litopenaeus vannamei. To the above, a bioassay, with treatments in triplicate, was performed to test different additive concentrations. WSSV analysis was done in organisms infected with a relatively high viral load using single and nested PCR. At the end of the bioassay, hemolymph was extracted and the immune system was studied in hemocytes at biochemical and genetic level (quantitative PCR). The final growth was similar in all treatments and survival was between 66,7% and 93,3%. WSSV prevalence decreased 13% as compared to control. The number of hemocytes, phenoloxidase activity, and superoxide anion concentration were similar in all treatments. Inulin and fulvic acid modulated the expression of transglutaminase, superoxide dismutase, and prophenoloxidase genes, but not the Toll receptor. Additives do not negatively affect growth of white shrimp and they protect them against WSSV when infected with a relatively high viral load. Additives did not affect the immune system effectors studied at biochemical level but they modulated the expression of some immune-related genes in L. vannamei.